Method for making ferritic stainless steel sheet having excellent workability

ABSTRACT

A method for making a ferritic stainless steel having an excellent workability, particularly bending workability and producing almost no ridgings, comprising annealing a hot-rolled steel band at a high temperature above 900* C., quenching the annealed steel band to form a martensite, then quenching the steel to a temperature below 760* C. so that the martensite with not be decomposed and thereafter subjecting the steel to conventional cold-rolling and annealing.

United States Patent Watanabe et al.

[54] METHOD FOR MAKING FERRITIC Apr. 4, 1972 2,851,384 9/1958 Waxweiler ..l48/l2 STAINLESS STEEL SHEET HAVING 3,067,072 12/1962 Leffingwell et al. 148/12 EL 3,128,211 4/1964 Waxweller ..148/12 EXC LENT WORKABILITY 3,139,358 6/ 1964 Graziano ..148/12 [72] Inventors: Shozo Watanabe; Takayuki Ooka; Susumu 3,141,300 1 964 Reichenbach 8/ l2 Takemura; Mothohiko Arakawa, all of 3,309,238 3/1967 Randak et all 148/ 12 Hika i Ci Japan 3,490,956 l/1970 Wilton ..148/12 [73] Assignee: Nippon Steel Corporation, Tokyo, Japan Primary Dewayne Rutledge [22] Filed; Oct 2 1968 Assistant Examiner-W. W. Stallard Attorney-Wenderoth, Lind & Ponack [21] App]. No.: 770,341

[57] ABSTRACT [52] US. Cl. ..148/l2.4 A method for making a ferritic stainless steel having an excel- [51] Int. Cl .C21d 1/32, C21d 9/48 lent workability, particularly bending workability and produc- [58] Field of Search ..148/12.4, 12 g almost no ridgings, comprising annealing hot-rolled el band at a high temperature above 900 C., quenching the an- [56] References Cited nealed steel band to form a martensite, then quenching the steel to a temperature below 760 C. so that the martensite UNITED STATES PATENTS with not be decomposed and thereafter subjecting the steel to conventional cold-rolling and annealing. 2,772,992 12/1956 Kiefer et al. 148/12 2,808,353 10/ 1 957 Leffingwell et al. 148/ 12 3 Claims, 6 Drawing Figures 6 10 A 500 E 0- I 3 phnse g' a 7 400 g m s '5 e 300 E J 5 4 Impact value 200 3 E '3 I 2 100 Termpering Temperature ("Cl Patented April 4, 1972 3 Sheets-Shut l FIG.

FIG.-

IN VENT ORS lllafanabe Sho'zo Talrayuki Ooka Susumu Takemura BY Mofohiko Ara/rawa w a w 14w! I ATTORNEY Patented April 4, 1972' 3 sheets sheet 2 Shozo Ta/rayuki Susumu Mo/oh/ko INVENTORS lilafanabe Oaka Takemura Arakawa ATTORNEY! Patented April 4, 1972 1 Impact value (krj /cm 5 Sheets-Sheet 5 100 200 300 400 500 600 700' 800 Termpering Temperature (C) V ickers hardness (Hv) INVENTORS Wafanabe Takayuki 5oka Susumu Takcmura Mofohiko Arakawa vim/W ATTORNEY METHOD FOR MAKING FERRITIC STAINLESS STEEL SHEET HAVING EXCELLENT WORKABILITY BACKGROUND OF THE INVENTION 1. Field of Invention The present invention relates to a method for making a ferritic stainless steel sheet having excellent workability, particularly bending workability and producing almost no ridgings.

2. Description of the Prior Art In general, a ferritite series stainless steel sheet containing 16 to 18 percent Cr is mainly used for deep drawing applications when it is a thin steel sheet having a thickness of less than 1.0 mm, and for bending applications when it has a thickness of more than 1.0 mm. In a ferritic stainless steel sheet repeated one-directional rollings in a continuous rolling system usually produce ridgings in the direction parallel to the rolling direction on the surface of the steel sheet, when the steel sheet is subjected to a deep drawing cracking is also produced in the direction parallel to the rolling direction at the part of the steel sheet which is subjected to a bending operation. They are faults of the gravest nature.

A so-called 17 percent Cr stainless steel containing 17 to 18 percent Cr, 0.03 to 0.12 percent C, less than 1 percent Si and less than 1 percent Mn, the rest being Fe and unavoidable impurities is for the most part the a phase of the body-centered cubic system and to a degree of to 50 percent the 'y phase of the face-centered cubic system even at a temperature above 900 C., depending upon the composition.

Therefore, this kind of steel is called a ferritic steel. When hot-rolling the ferritic steel, there are formed a rolling texture (1 00) 01 l of the a phase and a 01 l fibrous texture having a crystalline orientation rotated around an axis which is 01 1 parallel to the rolling direction (R.D.), whatever the finishing temperature may be in the range of the a transformation temperatures (see FIG. 1). It has been confirmed that the workability, particularly the degree of ridging and the bending workability of a steel sheet prepared by cold-rolling and annealing the above-mentioned hot-rolled steel sheet are closely connected with the presence or absence of this 01 l fibrous texture.

SUMMARY OF THE INVENTION The main object of the present invention is to provide a method for making a ferritic stainless steel having an excellent workability, particularly bending workability and producing substantially no ridging by removing from a hot-rolled steel band (100) 01 1 texture and 01 l fibrous texture parallel to the rolling direction.

Other objects of the present invention will be made clear by the following explanation with reference to the attached drawings.

According to the method of the present invention the starting material is a hot-rolled steel band prepared by a conventional steel-making method, wherein a molten stainless ingot obtained by a normal refining process is subjected to ingotting, blooming and hot-rolling. The thus obtained hotrolled steel band is annealed at a temperature above 900 C. and then quenched at a cooling speed at least as great as that of a cooling by air to obtain a martensite structure thereby. The steel band consisting of the martensite structure is then tempered at a temperature below 760 C., at which temperature decomposition of the martensite structure does not occur, and is then subjected to cold-rolling and annealing usually used in making stainless steel strip. The said cold-rolling may be carried out in a single step or multiple steps. However, when the hot-rolled steel band is annealed and tempered at a temperature below 760 C., as specified by the present invention, a stainless steel sheet of a relatively heavy gauge having an excellent bending workability can be obtained by a coldrolling in single step at a relatively small reduction.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is the (200) pole figure of the central part of a steel band after it has been hot-rolled;

FIG. 2 is the (200) pole figure of the central part of a steel band after being hot-rolled and then annealed at 815 C. for one hour;

FIG. 3 is the (200) pole figure of the central part of a steel band after being hot-rolled and then annealed at 815 C. for one hour and thereafter cold-rolled with the reduction of 60 percent and then again annealed at 840 C. for 10 minutes;

FIG. 4 is the (200) pole figure of the central part of a steel band after being hot-rolled and then annealed at 980 C. for 2 minutes;

FIG. 5 is the (200) pole figure of the central part of a steel band after being hot-rolled and then annealed at 980 C. for 2 minutes and thereafter cold-rolled with a reduction of percent and then again annealed at 840 C. for 10 minutes;

FIG. 6 is a diagram showing changes in the impact strength and the Vickers hardness of the martensite (a') phase of a steel sheet at various temperatures at which the said steel has been tempered for one minute respectively after being hotrolled and then annealed at 980 C. for 2 minutes and thereafter cooled in air.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the following the features of the present invention will be clarified by explaining stepwise various structures shown in the drawings above-mentioned.

It is evident from FIG. 1 showing the (200) pole figure of a hot-rolled steel band that the texture of the hot-rolled steel band consists of 0l1 and 0ll fibrous texture. When this hot-rolled steel band is annealed for softening at 750 to 850 C. as usual, the texture does not clearly show a definite aggregation, and the 0l l fibrous texture is still present to a relatively high degree, as is seen from FIG. 2. In the texture obtained when the steel band hot-rolled at a reduction of 60 percent and thereafter again annealed, the main orientation is made of the rotating system, the axis of which is lll parallel to the direction normal to the sheet plane (ND) and there remains still the 0l l fibrous texture which was originally present in the hot-rolled steel band as seen in FIG. 3. In the steel sheet thus cold-rolled and annealed, which has the texture as above-mentioned, a ridgin g is very often formed and the workability is reduced, because crackings occur when it is subjected to a bending working.

In order to eliminate the (100) 01 l and 01 l fibrous texture of a hot-rolled steel band, it is necessary to anneal the steel band at a temperature above 900 C., at which temperature a part of the structure is transformed to the 7 phase, and by doing so, such (100) 01l and 01 1 fibrous texture is eliminated and the recrystallization texture is obtained, as shown in FIG. 4. It has been revealed, however, that only by annealing of the hot-rolled steel band in the 7 range is it impossible to obtain a random orientation in the steel sheet after it is finally heat-treated, as will be explained hereinafter. The random texture can obtained, as is shown in FIG. 5, when the hot-rolled steel band is annealed at a temperature above 900 C. and then quenched and thereafter cold-rolled at a reduction of 90 percent and finally annealed at 800 to 940 C., whereby there can obtained a steel sheet which has substantially no ridging and such an excellent workability that a bending radius of 0 can be obtained, that is, a tight bending is feasible. This random orientation is very important for avoiding ridging, and in order to obtain such a random structure it is necessary to transform the 7 phase to a hard martensite (a' phase) by quenching the hot-rolled steel band annealed at a temperature above 900 C. When subjecting a ferritic steel containing the hard a phase to a cold-rolling, there is shown a remarkable peculiarity in the manner of transformation, that is, then occurs, particularly in the vicinity of the grain boundry of the a phase, a high stress state and the formation of nuclei at the time of a recrystallization annealing is different from that usually perceived, whereby the random orientation can be obtained.

However, in a ferritic stainless steel containing the a phase in an amount of 10 to 50 percent, usually 20 to 30 percent, ob

tained by quenching a hot-rolled steel band at a cooling velocity such as that of cooling in air, after it was annealed at a high temperature such as in the 7 range, the toughness thereof is so low that it is very difficult to cold-roll the same. In view of this fact it is conventional to cool the steel so slowly that the a phase is not be formed, based on the idea that the presence of the a phase would be the cause of the embrittlement, or to anneal the steel at a temperature of 750 to 900 C to cause the a phase to decompose and soften after it has been quenched, thereby to secure a sufficient toughness to be able to be easily cold-rolled. However, as above-mentioned, the thus softened, decomposed a phase, which is not able to be distinguished from the ferrite base, is not at all effective to destroy the fibrous texture and to aid in the formation of the random orientation during the final steps of cold-rolling and annealing.

The present invention has as an object to provide a heattreating method, wherein the toughness of the steel can be recovered in the hard state without substantially decomposing the effective a phase.

The steel to be treated by the method of the present invention is a ferritic stainless steel, the chemical composition of which is as follows: less than 0.15 wt.%, preferably 0.03 to 0.12 wt.% C, less than 1.0 wt.% Si, less than 1.0 wt.% Mn, 13.0 to 25.0 wt.%, preferably 14 to 18 wt.% Cr and the balance being Fe and unavoidable impurities. For producing the above-mentioned steel sheet any conventional method may be used. A ferritic steel having the composition as above-mentioned is usually hot-rolled to a steel band of a proper gauge and thereafter subjected to cold-rolling and annealing respectively one time or several times as determined to make a steel sheet of the final gauge. On the other hand, the method of the present invention is characterized in that the hot-rolled steel band is annealed at a high temperature above 900 C and then 35 quenched at a cooling velocity at least as great as that of a cooling by air to form martensite thereby and thereafter the thus quenched steel band is tempered at a temperature below 760 C. 1n the said annealing at a high temperature above 900 C., however, the temperature should not be too high, otherwise the melting of carbide into the steel would be increased and the coarsening of crystal grains would be effected, whereby the martensite structure would not formed in a desired distribution by the subsequent quenching. Consequently, the upper limit of the annealing temperature should not exceed l,l C. The tempering which follows the said quenching is to recover the toughness of steel. However, it should be properly selected in the range below 760 C. taking structure will be caused, but the object of the present invention can also be achieved, if the decomposition thereof would be caused to such a small degree that the hardness (Hv) of the a phase is still above 350. Further, this treatment is carried out for a short time if the temperature is at the higher end of the range, and vice versa. The martensite phase is completely decomposed, if the tempering is carried out for more than 2 hours at 750 C. and for more than 5 hours at 650 C.

1n the method of the present invention the tempering is usually followed by cold-rolling in a single or multiple steps. In

the case of the multiple cold-rolling an intermediate annealing is carried out. The final annealing which follows the said coldrolling, is carried out below 860 C. If a steel sheet is treated by the method of the present invention as above-mentioned, a

steel sheet having an excellent bending workability can be obtained even with a relatively thick steel sheet obtained by a single cold-rolling.

FIG. 6 shows the relations between tempering temperature and toughness (Charpy impact value) and Vickers hardness,

that is, the hardness (Hv) of the a phase. The sample used for these tests was a hot-rolled steel band 4mm thick containing 0.06 percent C, 0.45 percent Si, 0.5 percent Mn, 17.0 percent Cr and the balance being substantially Fe.

As is seen from this figure, the maximum impact value is obtained by a tempering for one minute at 750 C and the hardness of the or phase declines sharply from this temperature up. This impact value is determined by the relation between temperature and time so that the maximum impact value can be obtained without causing a grave reduction in the hardness of the a phase, wherein the longer the time, the lower the temperature. Therefore, if a hot-rolled steel band is annealed at a high temperature above 900 C and then quenched at a cooling velocity at least as great as that of a cooling by air to form the a phase and then tempered at a temperature below 760 C., at which temperature the toughness of the steel is recovered without decomposing the a phase, there can be obtained a ferritic stainless steel sheet, which produces no ridging during working and has such an excellent workability that it is able to be bent with a small bending radius, because EXAMPLE 1 Examples of various treating methods and results obtained thereby are shown in Table l.

TABLE 1 Test No.

Hot-rolled steel sheet:

Annealing temperature/time Cooling.. 0) 0) Tampering temperatures C. 750 670 800 600 Hot-rolled and annealed steel sheet, Impact value (kgr /cmfi) 10. 5 0.9 10.8 11. 2 9. 5 9. 8 10. 1 10. 3 Cold-rolled and annealed steel sheet:

Thickness, mm 0. 8 0.8 0.8 0. 8 0.8 0.8 1.5 1.5 Ridging uneveness, 24-30 36 36 2-6 16-30 20-34 Bending radius. 0 0 0 0 0 0 O 0 Yield point, lrg/mm. 32. 3 33. 3 31. 8 31. 6 32. 5 32. l 30. 3 30.9 Tensile strength, kg./mm. 51. 4 52. 3 50. G 50. 5 51. I 52. 3 51.0 51.8 Elongation, percent 33. 6 34. 0 33.0 32. 9 32. D 33. 6 35. 6 36, 0

1 815 C./2 hr.

2 980 C./2 min.

3 Cooling in furnace. 4 Cooling by air.

into consideration the combined temperature and time so that the decomposition of the martensite structure will not be caused. In this case the treatment is preferably to be so In the tests set forth in the table a hot-rolled steel band having the same composition as that shown in FIG. 6 was used as a starting material. After annealing or tempering the sample selected that substantially no decomposition of the martensite 75 material was cold-rolled at a reduction of 40 to percent and then annealed at a temperature of 800 to 860 C. The test No. 1 was according to a conventional standard treating condition. The test No. 2 was one wherein the sample obtained by a batch rolling on a laboratory scale was annealed at a temperature as specified by the present invention, but was not subjected to a tempering. In the tests Nos. 3 to 8 the starting materials were obtained by a continuous rolling on an industrial scale, wherein Nos. 3, 4, 7 and 8 were those which met the treatting conditions specified by the present invention, while the test No. 5 was a method wherein the tempering was not carried out and the test No. 6 was one wherein the tempering was carried out in such a way that the a phase was decomposed. From the table it is seen that samples treated by the method according to tests Nos. 3, 4, 7 and 8 have an excellent ridging property and bending workability, while in tests Nos. 5 and 6 the effect of the high temperature annealing was not displayed.

EXAMPLE 2 Tests using stainless steel sheets having the same composition as Example 1 have been made to determine the ridging property and bending workability. In Tables 2 and 3 the method of the present invention sample was treated according to the testNo. 4 ofExample l,and in the conventional method sample was treated according to the test No. l of Example I.

The results of the tests are as shown in the Tables 2 and 3.

Table 2 Comparison of ridging property (plate thickness of 0.7 to 0.8mm)

Ridging Method of present Conventional unevenness invention method") 0 20 n 3571 13% Remarks: With regard to the method of the present invention the test was made on 20 samples. The test results are shown in 7:.

**) With regard to the conventional method the test was made on 92 samples.

Table 3 Comparison of bending workability (plate thickness of 15 to 30mm) Bending working Method of present Conventional invention") method Number of samples.

wherein no cracking is produced at high 88% 17% bending Number of samples wherein cracking is produced at tight I271 25% Remarks: In the method of the present invention the test was made on 16 sam ples. The test results are shown in 7r.

In the conventional method the test was made on I63 samples. the test results are shown in 7:.

As can be seen from Tables 2 and 3, the ferritic stainless steel sheets treated according to the method of the present invention are much superior to those treated according to the conventional method with respect to ridging property and bending workability.

The scope of the present invention is not limited to the examples as above-mentioned, but may involve any method which does not depart from the idea of the present invention.

What is claimed is:

l. A method for producing a ferritic stainless steel, in which substantially no ridging occurs and which has excellent workability, comprising carrying out on a hot-rolled steel sheet comosed of C in an amount less than 0.15 wt.%, Si in an amount ess than 1.0 wt.%, Mn in an amount less than 1.0 wt. and 13 to 25 wt.% Cr and the balance being Fe and unavoidable im purities, the steps of annealing the hot-rolled steel sheet at a temperature between 1,100 and 900 C., rapidly-cooling the annealed steel sheet at a cooling velocity at least as great as cooling by air to cause a martensite structure to be formed, and then subjecting the steel sheet to a subsequent heat treatment at a temperature between 760 and 350 C. for recovering the toughness and for a time sufficiently short that the martensite structure is not decomposed, and thereafter subjecting the thus heat treated steel sheet to at least one conventional cold-rolling and subsequent annealing.

2. A method as claimed in claim 1, wherein the steel is composed ofC in an amount less than 0.12 wt. 14 to 18 wt. Cr, Si in an amount less than 1.0 wt. and Mn in an amount less than 1.0 wt. and the balance being Fe and unavoidable impurities.

3. A method as claimed in claim 1, wherein the hot-rolled steel band is annealed at a temperature of from 900 to 1 ,100 C. 

2. A method as claimed in claim 1, wherein the steel is composed of C in an amount less than 0.12 wt. %, 14 to 18 wt. % Cr, Si in an amount less than 1.0 wt. %, and Mn in an amount less than 1.0 wt. % and the balance being Fe and unavoidable impurities.
 3. A method as claimed in claim 1, wherein the hot-rolled steel band is annealed at a temperature of from 900* to 1,100* C. 